1. Field of the Invention
The present invention relates generally to a global positioning system (GPS). More particularly, the present invention relates to a GPS receiver and method for compensating for Doppler variation to accurately detect GPS signals in an environment in which intensities of the GPS signals received from GPS satellites are very low.
2. Description of the Related Art
With the development of technology, personal portable communication is rapidly developing, and various supplementary services are being supported. Some countries have enacted laws stating that mobile terminals must be equipped with a global positioning system (GPS) device. It is a trend that various location-based services are providing to many mobile terminals. Many GPS satellites broadcast ephemeris and system time information while orbiting the Earth, such that GPS receivers can determine their positions. The GPS receivers can accurately determine their positions by computing the arrival times of GPS signals simultaneously transmitted from at least four satellites.
This procedure requires several minutes to determine a user's position. More specifically, a compact GPS receiver with a limited amount of battery power cannot perform the above-mentioned procedure for a prolonged period. Accordingly, some GPS receivers receive, from an assisted GPS (AGPS) server, basic Doppler information, that is, coarse code phase values and coarse Doppler values, necessary for a search. Multiple satellites must be able to simultaneously observe a GPS receiver, and the GPS receiver must receive high-quality signals from the satellites. Because many portable or mobile devices may not be equipped with high-quality antennas, and/or may be located within forested areas and buildings, it is difficult for the portable devices to receive high-quality signals.
A conventional GPS does not use a pilot channel in which no data bit is modulated, but can remove a data bit by making use of a predicted data bit provided from the AGPS server. An improved coarse acquisition (C/A) code of a L2 band can use unmodulated data. Because unmodulated signals can be coherently integrated for a long time, it is very important that reception sensitivity be improved through coherent integration of GPS signals with weak signal intensities. Application of an accurate Doppler frequency is essential in the time-consuming coherent integration. More specifically, because a frequency error generated from a local oscillator (LO) or a Doppler offset due to user motion lowers a correlation energy value in time-consuming coherent integration, it makes signal acquisition difficult.
FIG. 1 is a block diagram illustrating a conventional GPS receiver 100 for detecting a GPS signal. The GPS receiver 100 has a relatively compact structure such that it can be mounted to a mobile phone, remote communication device, or portable device.
Referring to FIG. 1, an antenna 102 receives a radio frequency (RF) signal from a GPS satellite, and a signal receiver 104 converts the received RF signal into an intermediate frequency (IF) signal. The signal receiver 104 converts the IF signal into a digital signal and then outputs the digital signal to a mixer 108. The mixer 108 mixes a carrier frequency signal with the digital signal and then outputs a result of the mixing to a correlator 120.
A carrier numerically controlled oscillator (NCO) 114 and a code NCO 116 compensate for a carrier phase error and a code phase error according to relative position or speed variation, respectively. The GPS receiver 100 can further include an AGPS receiver (not illustrated) for receiving coarse Doppler information and other signal parameters from an AGPS server. The Doppler information and the parameters received from the AGPS server are provided to the carrier NCO 114, a code generator 118, and the code NCO 116.
The carrier NCO 114 generates a carrier frequency signal appropriate for a Doppler search using an oscillation signal provided from a temperature-compensated crystal oscillator (TCXO) 112, and then provides the carrier frequency signal to the mixer 108. The code NCO 116 associated with the GPS satellite generates a code frequency signal in which phase has been corrected at a carrier frequency. The code generator 118 generates a pseudo random noise (PRN) code of the GPS signal in response to the code frequency signal. The correlator 120 correlates a signal output from the mixer 108 with the PRN code to obtain a correlated sample. Correlated samples are accumulated for approximately 1 msec, and a result of the accumulation corresponds to a result of 1-msec coherent integration.
A Doppler frequency generated due to relative motion between the GPS satellite and the GPS receiver 100 influences peak values of the correlated samples. This influence is not completely removed by the carrier NCO 114. Accordingly, the GPS receiver 100 controls the carrier NCO 114 to perform a Doppler search. That is, the carrier NCO 114 outputs the carrier frequency signal while varying the carrier frequency by a predetermined frequency offset within a predetermined Doppler search range. The correlated samples based on carrier frequency signals are stored in a memory 122 such as a random access memory (RAM).
A coherent integrator 124 reads samples from the memory 122, accumulates the samples by the number of coherent integrations, and coherently integrates the accumulated samples. A signal detector 126 detects a correlated sample with peak energy greater than a predetermined detection threshold value from correlated samples output by the coherent integrator 124. A carrier frequency with the peak energy is regarded as a Doppler frequency.
In an indoor environment in which GPS signals with sufficiently strong intensities cannot be obtained from GPS satellites, a GPS receiver of a mobile terminal is notified of a search range of a code and a Doppler frequency to be searched for from an adjacent AGPS server to improve reception sensitivity, and effectively detects low-level signals by increasing a coherent integration time using the Doppler frequency search range.
However, there are limitations to increasing the coherent integration time to improve the reception sensitivity. First, there is a problem in that correlation effect is reduced by a polarity of a navigation data bit in a GPS signal when time-consuming coherent integration is performed because the GPS signal, which is different from a code division multiple access (CDMA) signal, does not have a pilot channel. Second, there is another problem in that a signal-to-noise ratio (SNR) is reduced when time-consuming coherent integration is performed in a state in which Doppler frequency variation is not compensated for.
To address the first problem, the AGPS server sends in advance predicted message bits to a terminal through a separate network. A position of a data bit boundary of a 20-msec cycle can be synchronized by making use of a system time of a CDMA network with an accuracy of approximately several micro seconds (μsec).
The second problem can be addressed by predicting Doppler variation. Doppler variation is caused by the following three factors:
I) GPS satellite motion and the rotation of the Earth vary a Doppler frequency of a GPS signal according to time. Conventionally, the Doppler frequency has a variation rate in the range of 0.5 to 1 Hz per second.
II) A signal used in the terminal is generated according to a user's local clock, and the instability of the local clock varies a GPS signal.
III) The last factor is user motion. According to the user motion, the Doppler frequency has a variation rate in the range of 1 to 10 Hz per second. It is impossible for the user motion to be predicted. The user motion may significantly affect the Doppler frequency for a coherent integration time of less than 1 second, and may basically limit reception sensitivity.